1. Field of the Invention
This invention relates to an implantable sensor for making amperometric measurements in body fluids, such as blood or tissue liquid, with a measurement electrode. More specifically, the invention relates to a sensor that detects blood glucose levels in vivo and initiates the automatic dispensing of insulin if detected blood glucose levels exceed a desired set point.
2. Description of the Prior Art
A patient suffering from the metabolic disease diabetes mellitus must balance his or her blood sugar level several times daily by injection of insulin. In addition, blood sugar controls are required in order to protect the patient against substantial metabolic deviations. These blood sugar controls typically require pricking the patient's finger tip to draw blood, an inconvenience for all patients and a substantial burden for small children and adolescents.
In order to assist in the control of blood sugar, electrochemical enzyme sensors have been used to determine blood glucose in vitro. Many of these devices, however, are quite expensive and heavy in their construction, such that they can be considered only for stationary treatment programs.
The use of implantable enzyme sensors has also been proposed. See, e.g. Zier, et al., U.S. Pat. No. 4,919,141. A significant disadvantage of these and other enzymatic sensors is that they cannot be used continuously, as they rely on platinum electrodes which react too quickly with body fluids to offer continuous, long-term service. Consequently, implantable enzyme sensors have a useful life of relatively short duration.
Still another disadvantage of prior art sensors is the tendency for the measurement electrode to become coated with deposits of biological materials, which impede performance. This problem is especially acute in implantable units, which become less effective over time as a result of the encrustation of the electrode.
Parce et al., U.S. Pat. No. 4,911,794, discloses the use of semiconductive electrodes for analyte measurement using a zero volume cell. The patent discloses the use of a number of semiconductive electrodes, such as silicon and gallium arsenide, as well as gallium selenide or aluminum gallium arsenide as the working or sensor electrode. Parce et al. is not, however, concerned with an implantable unit for measuring blood glucose.
Lerner et al., U.S. Pat. No. 4,340,458, discloses an electrochemical glucose sensor having a glucose oxidation electrode and a glucose-permeable membrane that separates the electrode from high molecular weight compounds. The electrodes of this reference are teflon-bonded platinum and silver/silver-chloride. Neither would be acceptable for an implantable glucose sensor. Obtaining a glucose concentration dependent signal by use of a platinum electrode is unreliable, non-reproducible and non-specific, as platinum reacts with almost anything. Furthermore, platinum does not oxidize or reduce glucose, rather oxidizes and reduces phosphate in the presence of glucose.
Cerami, U.S. Pat. No. 4,436,094, discloses a method for continuous, in vivo measurement of glucose in body fluids, such as blood, using a glucose monitor including an electrode with a charge-transfer medium comprising a reversible complex of a binding micromolecular component and a charge-bearing carbohydrate component. The electrode is enclosed in a semi-permeable membrane, selectively permeable to glucose.
Bombardineri, U.S. Pat. No. 4,633,878, discloses an implantable device for automatic insulin or glucose infusion in diabetic subjects, based on the continuous monitoring of the patient's glucose levels. The device uses an enzymatic-pontentiometric glucose sensor. The reference also discloses the use of hollow fibers, which form a filter through which only low molecular weight molecules may pass.
In spite of the above teachings, there is a need for an implantable electrochemical sensor capable of infusing medicines, such as insulin, when needed.